Distinct expression patterns of polycomb oncoproteins and their binding partners during the germinal center reaction

Polycomb group (PcG) genes encode two chromatin-binding protein complexes, the PRC1
and the PRC2 PcG complexes, which are essential for the maintenance of cell identity and
play a role in oncogenesis. PcG complexes were recently identified as novel regulators of
hematopoiesis, and appear to be expressed in a non-overlapping pattern in resting and
mature follicular B cells. Using highly specific antisera in combination with immunohistochemistry and triple immunofluorescence, we investigated the expression pattern of nine human PcG genes in germinal center (GC) B cells and highly purified germinal center B cell subpopulations. PcG proteins were detected in characteristic binding patterns that were not necessarily related to mutually exclusive expression of the two PcG complexes. We conclude
that the two PcG complexes are expressed throughout GC development, and that the
fine composition of each complex is determined by the differentiation status of the cell. In
addition, a subset of dividing cells with a centrocyte CD marker profile was identified that coexpresses core components of the PRC1 and PRC2 complex. We propose that these cells
reflect a transitional stage between resting and dividing follicular B lymphocytes, and that
they possibly represent the healthy precursors of nodal large B cell lymphomas.